1. Field of the Invention
The present invention relates to a charging circuit for charging a secondary cell housed in an instrument such as a cordless hand set (slave hand set) of a cordless-phone (cordless telephone system).
2. Description of Related Art
In the cordless-phone, the cordless hand set (called a "child machine" hereinafter) comprises a secondary cell type battery, and operates with receiving an electified power from the battery when it is in use. This child machine is set on a charging stand when it is in a stand-by condition, so that the battery is charged by an external DC power supplied from this charging stand.
First of all, a conventional basic charging circuit widely used for the cordless hand set of the cordless-phone comprises a pair of external DC power supply terminals to be connected to the circuit of a charging stand of the child machine when it is set on the charging stand. These external DC power supply terminals are connected to the battery housed in the child machine through a diode and a charging resistor. Thus, if the child machine is set on the charging stand, the battery is charged by the external DC power, supplied through these external DC power supply terminals. The charging resistor is provided to limit the charging current of this battery, thereby facilitating regular charging in the charging circuit.
A main power supply switch and a power supply circuit are connected to the battery in parallel with the external DC power supply terminals, the diode and the charging resistor. The power supply circuit is intended to supply, on the basis of a voltage supplied from the battery, a V.sub.DD voltage to a control circuit for controlling the operation of the child machine. The main power supply switch is provided as a switch for operating a turn-on and a turn-off of the power supply circuit. Thus, this charging circuit can charge the battery from the external DC power supply terminals even in the condition that the main power supply switch is turned off to stop the operation of the child machine.
A conventional charging circuit which performs a rapid charging function comprises a rapid charging switching circuit instead of the diode and the charging resistor in the above mentioned basic charging circuit. The rapid charging switching circuit is formed by connecting a series circuit of a diode and a charging resistor and another series circuit of a PNP transistor and another charging resistor in parallel to each other. The first named charging resistor is the same as that of the above mentioned basic charging resistor and effects the usual charging. The second named charging resistor is a resistor for a rapid charging, having a resistance smaller than that of the first named charging resistor. Further, the turn-on and turn-off of the transistor is controlled by a rapid charging signal supplied from a control circuit, which monitors the voltage of the battery by a detecting circuit. When the voltage is low, the control circuit generates the rapid charging signal which turns the transistor on.
Thus, when the child machine is set on the charging stand, this charging circuit supplies a charging current from the external DC power supply terminals through the second named charging resistor having the small resistance value to the battery in the case that the transistor of the rapid charging switching circuit is conductive, so that the charging current becomes a large current and can effect the rapid charging. Then, if the transistor of the rapid charging switching circuit is turned off, the charging current from the external DC power supply terminals 1 and 1' is supplied to the battery through the first named usual charging resistor and the usual charging is effected.
However, in the charging circuit as mentioned above, for example when the battery is removed, even if the external DC power supply voltage is supplied from the external DC power supply terminals, the external DC power supply voltage is supplied to the power supply circuit through the first named charging resistor or the second named charging resistor of the rapid charging switching circuit. Therefore, the conventional charging circuit could not supply, directly form the external DC power supply terminals, a sufficient driving current for operation of the control circuit 4.